This invention relates to position determining systems, such as digitizer tablets, and to pointing devices of such tablets, and in particular to digitizer tablets in which the pointing device typically in the form of a stylus or cursor is free of any cable or other electrical wire connection to the tablet itself.
Digitizer tablets employing a cordless pointing device, e.g. a stylus or cursor, for simplicity referred to herein as a stylus, are already known in the art. The major advantage is that the absence of the cable or electrical cord connection between the stylus and the tablet facilitates its use by the user. In a cordless system, one way of conveying information to the tablet is by way of transmitted electrical signals that induce signals in the conductors of the typical tablet grid that can be processed and interpreted by the tablet. The information can also include the status of any switches within or on the stylus. Information concerning the status of different switches could be conveyed with electrical signals at different frequencies. could be conveyed with electrical signals at different frequencies.
A digitizer tablet in use is typically connected to a host computer such as a PC which includes a monitor. The monitors of such computers radiate signals that may interfere with the operation of the digitizer tablet, particularly a digitizer tablet that uses electromagnetic or electrostatic coupling of the stylus to the conductors of the grid structure in the tablet.
To accommodate a stylus which uses a number of frequencies to convey switch status or other information, the tablet signal processor must have a relatively wide bandwidth, which makes the tablet more prone to noise and electromagnetic interference compared to a tablet processor with a narrow bandwidth.